In the ink-jet printing process, tiny droplets of ink-jet ink are applied directly to the substrate to be printed, without physical contact between the printer and the latter. Placing the individual droplets on the substrate to be printed is done under electronic control. Ink-jet printing is an important technology for printing on the one hand various substrates such as for example paper, card, film, foil, etc. or on the other products such as for example bottles, cans, etc. with diverse data or images. Ink-jet prints can even be provided to fragile objects, for example eggs. Ink-jet technology is capable of providing high definition prints.
Various droplet formation processes have been developed in the past. Droplet formation can be effected for example electrostatically, magnetically, piezoelectrically, electrothermally, via mechanical microvalves or by spark discharge. Irrespective of the manner of droplet formation, ink-jet technology divides in principle into two categories, into continuous ink-jet (CIJ) and impulse or drop-on-demand (DOD) technology.
In continuous ink-jet, ink flows under pressure through a nozzle to form droplets which are applied to a substrate in a continuous jet.
In impulse ink-jet, by contrast, the ink reservoir is kept at or below atmospheric pressure. A droplet of ink is only released from a nozzle in response to the droplet-forming unit being subjected to controlled stimulating impulse. This technology is mainly used in color printers.
After the droplets of ink-jet ink have been applied to the substrate to be printed, the ink-jet ink is dried or cured. This can be effected for example through agency of heat or UV light. This process ensues immediately upon application of the ink on the substrate to be printed. Drying or curing takes between fractions of a second and minutes depending on the drying or curing process.
Color printers have recently been developed to produce colored prints in the primaries yellow, cyan, magenta and black. These colors are widely used as process colors in established printing processes such as lithography, gravure printing or flexographic printing.
To further enhance the quality of colored prints, particularly in view of increasing uses of promotional prints, spot colors are used. These spot colors consist of a straight or a mixed ink and are printed in a single pass. These spot colors, in addition to the four primary colors, may be inter alia the colors orange, green or any further color to increase the color scale and the vitality of the printed image.
If the colored print is to be endowed with a metallic effect, metallic effect pigments can be added to the ink. To obtain a gold- or silver-colored print, gold- or silver-colored metallic effect pigments are used in the ink-jet ink. Useful gold-colored metallic effect pigments include for example brass pigments obtained from a copper-zinc alloy and known as golden bronzes. In brass pigments, the hue of the alloy is determined by the copper/zinc ratio. Golden bronze pigments are commercially traded in characteristic natural hues as “pale gold” having a copper fraction of about 90% by weight, remainder zinc, as “rich pale gold” having a copper fraction of about 85% by weight, remainder zinc, and as “rich gold” having a copper fraction of about 70%, remainder zinc. Alternatively, golden-colored prints are also obtainable by mixing silver-colored metallic effect pigments with yellow and optionally red colorants.
Platelet-shaped aluminum pigments for example are useful as silver-colored metallic effect pigments. WO 2009/010288 A2 discloses ink-jet inks containing thin aluminum pigments. Ink-jet inks containing metallic effect pigments are commercially available under the trade name of Jetfluid (from Eckart GmbH).
If it is not a purely golden- or silver-colored ink-jet print that is desired, but a metallically seeming printed image of a certain color, the ink-jet ink may have added to it, in addition to the metallic effect pigments, further colorants in the desired color to be achieved. Alternatively, the ink-jet inks containing metallic effect pigments can be printed together with further ink-jet inks present in the printing system (process inks) and thus be mixed therewith on the substrate to be printed. Depending on the desired effect to be achieved, it is initially also possible to print an ink-jet ink containing metallic effect pigments and subsequently to overprint it with a process ink.
EP 1 554 345 B1 describes aqueous ink compositions for the ink-jet printing process which provide prints having an appearance which is metallic or dependent on the viewing angle. The pigments in the ink composition comprise a core consisting of a transparent or metallically reflective material, and at least one coating consisting of one or more silicon oxides (SiOx layer). The average particle diameter is at least 2 μm.
DE 19727767 A concerns radiation-curable ink-jet inks containing at least one finely granular organic or inorganic pigment. Interference pigments are mentioned as examples of inorganic pigments. It is preferable for 95% and more preferable for 99% of the pigments to have a particle size <1 μm.
A common method of achieving a pearl luster effect in a printed image is to print on a substrate already precoated with pearlescent pigments, as described in JP 2003080836A, for example. However, in using such a substrate to be printed, it is disadvantageously not possible to provide the substrate with the pearl luster effect in selected areas only.